1. Field of the Invention
The present invention relates to a power supply device for a high intensity discharge bulb. In particular, the present invention relates to a power supply device for a discharge tube called a high intensity discharge bulb (hereinafter referred to as a HID bulb) for use in vehicles, such as a headlamp for an automobile or motorcycle.
2. Description of the Related Art
In HID bulbs, there are variations in drive voltage from bulb to bulb. For example, the drive voltage varies from 70 (V) to 100 (V), i.e., the drive voltage is 85±15 (V), which means that the drive voltage varies by about 18%. The emission intensity of a HID bulb is proportional to the electric energy to be applied, i.e., the amount of work (energy). Since HID bulbs have such characteristics, by controlling the currents of individual HID bulbs, the electric energy to be supplied is made constant and variations between products are reduced, whereby uniform intensity is maintained.
In order to control the driving electric energy of a HID bulb, there exists a method in which the pulse width is increased or decreased. This pulse width control method includes an analog type method and a digital type method. In the analog type method, an analog circuit in which pulse width modulation is performed is employed and a dedicated IC is used. In the digital type method, the pulse width is changed by counting the number of pulses and a general-purpose microprocessor is used in many cases.
FIG. 3A is a waveform diagram for explaining the principle of a “pulse counting system.” In this example, a duration of 17.5 μS, which is equivalent to 70% of a rectangular wave signal in a repetition period of 25 μS (40 KHz), is an “ON” time and a duration of 7.5 μS, which is the rest of 30%, is an “OFF” time.
The physical definition of electric power is the “power,” i.e., the work per unit time, and is represented by the unit of joule/second (J/S). Therefore,electric power(W)=voltage(V)×current(A)  (1),andelectric power(J/S)=voltage(J/C)×current(C/S)  (2),and accordingly, the supply energy (work) to a HID bulb is expressed by the following relational expressions:work(J)=power(J/S)×duration(S)  (3)andelectric energy(J)=electric power(W)×duration(S)  (4).
By applying a voltage of 85 (V) to a 35 (W) HID bulb and passing a current of about 0.412 (A), a predetermined electric power is supplied. Strictly speaking, since the signal is a rectangular wave and alternating-current driving signal, the power factor needs to be taken into consideration; however, in accordance with customary practice, it is represented by an effective value equivalent to a direct current.
Bulb 35(W)ON time17.5 μS (70%)OFF time 7.5 μS (30%)Period25.0 μS (100%) (40 KHz)
In this example, an electric power of 35 (W) is applied during an ON time of 17.5 μS, and thus, the electric energy is as follows:35(W)×17.5(μS)=612.5×10−6(J).
If the signal is sampled 200 times during a period of 25.0 μS (1 Time Slot (hereinafter referred to as “1 TS”)) on a time base, the each sampling time corresponds to 0.125 μS and an ON time of 17.5 μS has 140 samples. Since 35 (W) is 17.5 μS, 35 (W)/140=0.25 (W)/sample, that is, energy to a HID bulb is supplied by an electric power (power) of 0.25 (W) per pulse.
With this sampling pulse using as a unit, by increasing or decreasing an ON TIME of 17.5 μS, the electric energy, i.e., energy, can be controlled. In this example, the electric energy can be controlled in steps of 0.25 (W).
If the number of samples of 25.0 μS during the 1 TS, is reduced from 200 to 50, each sampling time corresponds to 0.5 μS and an ON time of 17.5 μS has 35 samples. 35 (W)/35=1.00 (W)/sample, that is, energy per each pulse is 1 (W), and thus, the step is raised four times the aforementioned case.
As can be seen from the above-described example, in the pulse counting system, the control accuracy may change depending on the number of samples.
To increase the accuracy, the driving frequency of a HID bulb needs to be increased to a sufficiently high frequency.
As a pulse counting means, a general-purpose microprocessor is used. The driving clock is about 30 MHz. By dividing the driving clock, above-mentioned sampling signals are obtained. These days, a small and low-cost component, called a PIC (peripheral interface controller), is supplied to the general-purpose microprocessor. However, the driving clock frequency of a PIC is as low as 10 to 20 MHz, resulting in a drawback such as low accuracy in the conventional pulse counting system.
For a drive device for a high intensity discharge lamp, there are known documents such as the one described in the “Ballasts for HID Head lamp Systems for Automobile” by Tsutomu Shiomi, Takashi Kambara, et al., Matsushita Electric Works Technical Report (May 2001), pp. 13-19.